Superconducting quantum interference devices (SQUIDs) are very sensitive detectors of magnetic flux. It is known to use a DC SQUID in a flux-locked loop as a null detector of magnetic flux. Typically a DC SQUID is exposed to a flux to be detected, and is simultaneously modulated with an alternating flux of some reference frequency. If there is no flux to be detected, the output signal from the DC SQUID resulting from the modulating flux contains only a signal of twice the reference frequency. If, however, there is flux to be detected, the DC SQUID output signal approaches closer to the reference frequency, with the polarity of the output signal varying with the polarity of the flux to be detected. A lock-in detector at the reference frequency and phase will provide an output voltage proportional in amplitude and equal in polarity to the flux to be detected. Locking of the detector reference frequency to the frequency and phase of the modulation flux results in a flux-locked loop.
If a square-wave modulation and demodulation process is employed, then the following equations hold:
for the modulation process, s.sub.m (t)=s.sub.s (t).times.m(t) PA1 for the demodulation process, s.sub.d (t)=s.sub.m (t).times.M(t) PA1 s.sub.s (t) is the DC SQUID output signal, PA1 s.sub.m (t) is the modulated signal, PA1 m(t) is a square wave modulating signal, PA1 s.sub.d (t) is the detected signal, and PA1 M(t) is the demodulation signal proportional to M(t).
where
Because M(t).times.m(t) is a constant, the detected signal is proportional to the output signal of the DC SQUID. In the conventional approach, the modulated signal is distorted and the original signal cannot be reconstructed completely.
Because the DC SQUID, operated at 4.2 degrees Kelvin, has a low output resistance (typically a few ohms) and since the input resistance of the room temperature operated pre-amplifier is on the order of several thousand ohms, some form of impedance matching is necessary. Typically, the impedance matching is accomplished via a coldside transformer or resonant LC circuit. In addition, these impedance matching networks also provide a limited amount of signal gain, so as to improve the signal-to-noise ratio associated with the transmission line used to couple the signal from the DC SQUID to the warmside preamplifier. However, the use of these impedance matching networks severely limits the bandwidth of the DC SQUID flux-locked loop (FLL).